As a chair having a relatively simple structure, there is a chair in which a backrest is attached to a frame device including bars or pipes. The chair of this type is allowed to have a rocking function so that the backrest is tilted backward by the leaning of a person sitting on a seat. An example thereof is described in Patent Document 1.
That is, in Patent Document 1, a frame device of a chair includes right and left lower horizontal portions which elongates in back and forth directions and placed on a floor, right and left front legs which rise toward the upper side from front ends of the lower horizontal portions, upper horizontal portions which extend to the rear side from upper ends of the right and left front legs, and right and left rear legs which rise from rear ends of the lower horizontal portions, and a seat is attached to the right and left upper horizontal portions.
Rear ends of the right and left upper horizontal portions rise toward the upper side and the right and left upper horizontal portions are integrally connected to each other at a horizontally long lower portion. The horizontally long portion is connected to a lower back face of a backrest. Further, the upper ends of the right and left rear legs are connected to each other by a horizontally long upper portion and the horizontally long upper portion is connected to a back face of a mid-height portion of the backrest. The backward tilting of the backrest of the chair in Patent Documents 1 is realized because the rear legs resist elasticity and are thus tilted backward, mainly.
Meanwhile, Patent Document 2 describes an example of a rotating chair which has a leg column formed of a gas cylinder and in which a seat and a backrest are supported by a frame device. Further, in a chair disclosed in Patent Document 3, a frame device has a shape opened toward the rear side in a side view and is fixed to a base, and a backrest is attached to a rear portion of the frame device.
Patent Document 1: Japanese Patent Publication No. 8-15446
Patent Document 2: Japanese Translation of PCT Application No. 9-502631
Patent Document 3: Japanese Registered Design Publication No. 390617